Unsaturated polyester foams have been prepared in the past by a number of different techniques. However, these processes have not been wholly successful or commercially feasible for various reasons. For example, a difficulty encountered in attempts to produce unsaturated polyester foams is to generate gases so as to cause a uniform expansion of the resin at ambient temperatures before any appreciable crosslinking occurs. The significant portion of the crosslinking and curing must ideally occur very soon after the maximum amount of gases has been released. Should appreciable crosslinking occur before maximum gas release, the accompanying exothermic reaction will cause cracking as the previously unreleased gases are further generated and cause stresses against a very rigid crosslinked structure which is unable to further expand. Should appreciable crosslinking not occur after maximum gas release has occurred, the gases will gradually escape and the expanded resin will gradually drop back to its original state. As a result, curing will occur much like a standard polyester resin casting or laminate with little or no expansion.
Some of the methods used in the past have utilized lower density additives such as polystyrene. However, these types of systems are limited in that they provide foams of only medium to high densities, i.e. .about.25 lb/ft.sup.3 (0.40 g/cc) or higher. The preparation of unsaturated polyester resin foams utilizing a hydrazide blowing agent in combination with peroxides is described in U.S. Pat. Nos. 3,920,589 and 3,920,590. Similar processes utilizing acid sensitive azo compounds are described in U.S. Pat. Nos. 4,009,157 and 4,029,615. Some of the limitations of such processes include the requirement of large percentages of accelerators, the use of either relatively high percentages of expensive azo blowing agents or the use of azo compounds of limited commerciability due to expense or difficulty of handling, i.e. azo compounds that require low temperature protection due to rapid room temperature decomposition and/or pose a hazard either directly or because of decomposition by-products.
Other publications describe the preparation of unsaturated polyester foams with the use of diisocyanates; see Chemical Abstracts Vol. 79, 116136j; Vol. 86, 141203f; Vol. 86, 172676s; Vol. 84, 91051r; and Vol. 86, 107410m. However, such processes require higher than ambient temperatures, long process times, limited density ranges, or high concentrations of the diisocyanates, amines, water, acids, carbonates and other accelerators which detract from the long term strength and stability of the cellular structure.